The subject invention is directed toward the art of chain power drives and, more particularly, to a chain guide and tensioner apparatus and related method useful in confined spaces for applying a tensioning force to a chain traveling therepast. The subject innovation provides a convenient snap-fit assembly taking advantage of a geometric interface between components to simplify installation.
While the invention is particularly directed to the art of chain power drives, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
Several prior art patents show chain tensioner devices that include a chain engaging shoe member formed from a plastic material with a metal spring or bracket incorporated therein to provide the shoe with the necessary resiliency and rigidity while taking advantage of the material's low friction and good wear characteristics. For example, my prior U.S. Pat. No. 4,921,472 discloses a chain tensioner wherein a blade spring is received in longitudinal passageways formed in a plastic shoe. The spring is retained in longitudinal position by internal shoulders in the passageways. Lateral positioning and retention is achieved by cooperation between longitudinal slots in the ends of the spring and central internal walls in the passageways.
Additionally, my prior U.S. Pat. No. 5,286,234 teaches a chain tensioner comprising an elongated shoe formed of a resinous plastic material and having first and second longitudinally spaced enlarged ends joined by a relatively narrow, resilient central body section. The shoe has a first exterior wall surface which extends the length thereof and is adapted to engage the chain to be tightened. Narrow slots extend laterally into the enlarged ends generally parallel to the first exterior wall surface. A blade spring having a length nearly as great as the shoe and terminating in first and second ends is received in the slot to extend longitudinally of the shoe generally parallel to the first exterior wall. A rigid metal support bracket is provided for mounting the shoe adjacent the chain to be tightened. The bracket has 1) a side wall engaging a side of the shoe and overlying the slots for retaining the blade spring in the slots, and 2) a pivot pin arrangement for retaining the shoe and blade on the bracket.
A consideration with my prior designs, as well as other prior art designs, is that it is desirable for spring length to be maximum. Increased spring length provides favorable results respecting working travel of the tensioner, which is the allowable angular, or vertical, deflection of the free end of the shoe/blade relative to the retained end, take-up capability, which is the ability of the tensioner to reduce or "take-up" any slack in the chain, and spring force throughout the life of the device.
Another consideration for most applications is that, typically, the whole tensioner, including the shoe and blade, is first installed on the engine block, and then the chain and sprockets are installed. For example, FIGS. 1 and 2 show an installation pin P holding the shoe S1 and blade B in proper position during installation. The pin P is then removed after the chain C and sprockets S2 are installed.
Another method of retaining the blade B prior to and during installation is to locate a pin or clip near the mid-point of the blade. While this mid-blade pin location serves to reduce the complexity of the blade, it also adds further to the complexity and cost of the bracket.
The pin P is used to prevent the profile of the combination of the blade B and the shoe S1 from intruding into the plane of the chain C as the chain C and sprockets S2 are installed. Additionally, the blade B and shoe S1 are retained by pin P to allow for the installation of the shortest possible chain. Moreover, the tolerances associated with the installed tensioner retained by the pin P affect the arrangement. Accordingly, the blade B and shoe S1 are retained in a position to accommodate both a sufficient clearance for installation and the "build stack-up", or accumulation, of component and fastener location tolerances.
An assembly of this type has reduced tension or take-up capability since the blade B must move into the chain C, once the pin P is removed, prior to any system wear taking place. Moreover, balancing all of these factors against the desire to use a blade (shoe) of maximum length, results in a chain tensioner that is shorter, somewhat more complex and, consequently, more expensive than is desirable, due to the necessity of accommodating an installation pin.